1. Technical Field
The present invention relates to an inkjet printer in which a plurality of nozzles jet minute ink drops of liquid ink of a plurality of colors and particles of the ink (ink dots) are formed on a print medium to draw pre-determined characters and images.
2. Related Art
An inkjet printer as in the above generally accomplishes low-cost and high-quality color printed material easily. As such, it is widely used not only in offices but also by general users along with popularization of a personal computer and a digital camera.
Generally, in such an inkjet printer, a moving part called a carriage, for example, integrally comprising ink cartridges and print heads moves back and forth on a print medium in a direction crossing a direction to convey the medium, and nozzles of the print head jet (eject) liquid ink drops to form minute ink dots on the print medium. In this manner, pre-determined characters or images are drawn on the print medium to create desired printed material. The carriage comprises ink cartridges for four colors including black (and yellow, magenta, cyan) and a print head for each of the colors, so that not only monochrome print but also full color print in combination of the respective colors can be easily performed (further, print in six colors including the colors, light cyan and light magenta, seven colors, and eight colors are practically implemented).
In the above type of inkjet printer for executing print by moving the inkjet heads on the carriage back and forth in a direction crossing a direction to convey a print medium in the above manner, the inkjet heads must be moved back and forth about ten times to more than tens of times to neatly print a whole page. Therefore, it has a drawback in that it takes a longer time for printing than a print apparatus in another scheme, for example, a laser printer or a copying machine using electrographic technique.
On the other hand, in an inkjet printer comprising inkjet heads (do not need to be integrated) of the same length as the width of a print medium but not comprising a carriage, the inkjet heads do not need to be moved in a width direction of the print medium so that one-pass printing is possible, enabling quick printing similar to a laser printer. An inkjet printer in the former scheme is generally called a “multi-pass (serial) inkjet printer”, while an inkjet printer in the latter scheme is generally called a “line head inkjet printer”.
The above types of inkjet printers are required to provide further higher gradation. Gradation is the density of each color included in a pixel represented by an ink dot: the size of an ink dot depending on the density of a color of each pixel is called gradient, while the number of gradients represented by an ink dot is called the number of gradations. High gradation means that the number of gradations is large. To change gradient, it is necessary to change a drive pulse to an actuator provided to an inkjet head. For example, if an actuator is a piezoelectric element, when a voltage value applied to the piezoelectric element is large, the magnitude of displacement (distortion) of the piezoelectric element (precisely, a vibrating plate) is also large. This is used to change the gradient of an ink dot.
According to JP-A-10-81013, a plurality of drive pulses having different voltage peak values are combined and coupled to generate a drive signal. The signal is output commonly to piezoelectric elements of nozzles for the same color provided to an inkjet head. According to the drive signal, a drive pulse for the gradient of an ink dot to be formed is selected for each nozzle. The selected drive pulse is supplied to a piezoelectric element of an appropriate nozzle to jet an ink drop. In this manner, a requested gradient of an ink dot is achieved.
A method for generating a drive signal (or drive pulse) is illustrated in FIG. 2 of JP-A-2004-306434. That is, data is read out from a memory for storing drive signal data, a D/A converter converts the data into analog data, and a drive signal is supplied to an inkjet head through a current amplifier. A circuit of the current amplifier comprises transistors in push-pull connection, as shown in FIG. 3 of the document, in which a linear drive amplifies a drive signal. However, in a current amplifier with such configuration, a linear drive itself of a transistor is inefficient. Moreover, such an amplifier has a drawback of a large circuit size since the transistor itself should be large for a countermeasure against heat, and the transistor needs a cooling plate radiator. Particularly, the largeness of the cooling plate radiator is a major obstacle to the layout.
To resolve the drawback, JP-A-2005-035062 discloses an inkjet printer for generating a drive signal by controlling the reference voltage of a DC/DC converter. According to the document, an efficient DC/DC converter is used to dispense with a radiating unit for cooling. Additionally, a PWM signal is used so that a D/A converter can be realized using a simple low-pass filter. These can realize a small circuit.
However, a DC/DC converter is originally designed to generate a constant voltage. As such, the head drive apparatus of an inkjet printer using the DC/DC converter in JP-A-2005-035062 has a problem in that a waveform, for example, rapid rise and fall of a drive signal cannot be gained necessary for an inkjet head to jet ink drops well. Of course, the head drive apparatus of an inkjet printer in which a pair of transistors in push-pull connection amplifies current of an actuator drive signal in JP-A-2004-306434 has a problem in that a cooling plate radiator is so large that it cannot be actually laid out particularly in a line head inkjet printer having a large number of nozzles, i.e., a large number of actuators.